Within the scope of the present invention, a bimetal part is understood as meaning a multi-layered active structural part in sheet form comprising two, three or four components with different coefficients of expansion connected inseparably to one another. The individual layers of metals or metal alloys are connected in a materially bonded or interlocking manner, achieved for example by rolling.
Such bimetal parts are commercially available as sheets, see for example the company G. Rau GmbH & Co. KG, Kaiser-Friedrich-Str. 7, 75172 Pforzheim, and their corresponding website at www.rau-pforzheim.de.
It is known in this connection from EP 0 658 911 B1 to use multi-layered bimetal parts as springs and discs in temperature-dependent switches, it being intended to achieve an increase in the possible nominal currents and switching hysteresis by appropriate material selection and composition.
The bimetal part is in this case part of a temperature-dependent switching mechanism which, depending on its temperature, closes or opens an electrically conducting connection between two fixed contact parts provided on the switch.
Such temperature-dependent switches are known in various designs from the prior art.
The bimetal part is in each case generally formed as a spring restrained at one end or a disc loosely inserted.
If the bimetal part is formed as a bimetal spring tongue as in DE 198 16 807 A1, it bears at its free end a movable contact part, which interacts with a fixed contact part. The fixed contact part is electrically connected to a first external connection, a second external connection being electrically connected to the restrained end of the bimetal spring tongue.
Below its response temperature, the bimetal spring tongue closes the electrical circuit between the two external connections by pressing the movable contact part against the fixed contact part.
If the temperature of the bimetal spring tongue increases, it begins to stretch and to deform in a creeping phase, until finally it springs over into its open position, in which it lifts the movable contact part off from the fixed contact part. In this creeping phase, the contact pressure is reduced, which may lead to the formation of arcs, contact erosion and contact chatter.
If, on the other hand, the bimetal part is designed as a bimetal disc, it generally interacts with a spring snap-action disc, which bears the movable contact part which interacts in the way described above with the fixed contact part. The spring snap-action disc is supported by its periphery on an electrode, which is connected to the second external connection. Such a switch is described, for example, in DE 21 21 802 A or DE 196 09 310 A1.
Below its response temperature, the bimetal disc is loosely inserted, is therefore not subjected to any mechanical load. The contact pressure between the fixed contact part and the movable contact part, and consequently the electrical connection between the two external connections, is provided by way of the spring snap-action disc. If the temperature of the known temperature-dependent switch increases, the bimetal disc passes through a creeping phase, in which it gradually deforms until it then suddenly changes over into its open position, in which it acts on the spring snap-action disc in such a way that it lifts off the movable contact part from the fixed contact part, and consequently opens the known switch. The creeping phase has no adverse effects on the contact pressure here.
In the case of the switch described above with the bimetal spring tongue, current flows through the bimetal part itself, so that it heats up as a result of the current flowing through the switch. In this way, the known switch not only reacts to external temperature increases, it also reacts to excessive current flow.
Such switches therefore react temperature-dependently and current-dependently.
By contrast with this, in the case of the switch with a bimetal disc, the bimetal part is always free from current; it is therefore not heated by the flowing current, so that such switches switch largely current-independently.
However, there are also known switches in which a bimetal spring tongue interacts with a spring snap-action part which carries the flowing current, so that in the case of these designs the bimetal spring tongue itself does not carry any current. Conversely, there are also known switches in which a bimetal disc bears the movable contact part and consequently has current flowing through it.
Finally, there are known temperature-dependent switches with two external connections, which are each connected to a fixed contact part, and provided with an electrically conducting contact bridge which carries the flowing current when it lies against the fixed contact parts.
Such switches with a contact bridge are described, for example, in DE 197 08 436 A1. They are intended for applications in which high nominal currents flowing through the switch would cause a current-carrying spring snap-action part or bimetal part to undergo great loading or self-heating.
The contact bridge is in this case carried by a spring snap-action disc, which interacts with a bimetal disc. If the bimetal disc is below its response temperature, it lies freely in the switch without any mechanical loading; the spring snap-action disc presses the contact bridge against the fixed contact parts, so that the circuit is closed. If the temperature increases, the bimetal disc snaps over from its force-free closed position into its open position, in which it works against the spring snap-action disc and lifts the contact bridge from the fixed contact parts.
Even in the case of this switch design, the aforementioned problems in connection with the creeping phase of the bimetal disc occur if it directly bears the contact bridge and provides the contact pressure. That is the reason why the known switch is provided with the spring snap-action disc, which maintains the contact pressure unchanged even in the creeping phase of the bimetal disc.
The switches described thus far are used for the purpose of protecting electrical appliances, such as for example hairdryers, motors for lye pumps, irons, etc., from excessive temperature and possibly excessive current. For this purpose, the known switches are connected with their external connections in series into the supply circuit of the electrical appliance to be protected and are also thermally coupled to the appliance to be protected.
If the temperature of the appliance to be protected increases beyond the switching temperature of the bimetal part, the temperature-dependent switch opens the circuit and the protected appliance can cool down again.
In order to prevent the appliance, and consequently also the bimetal part, from being switched on again after cooling down, it is also known to assign the temperature-dependent switch a shunt resistor, which, when the switch is open, allows through a residual current which heats up the resistor to the extent that the switch remains open. Such switches are referred to as self-holding switches.
It is also known to provide the known switches with a defined current dependence, by connecting in series with the external connections a heating resistor which is flowed through by the operating current of the electrical appliance to be protected and, when there is excessive operating current, heats up in a defined manner and ensures that the switch is opened, since the bimetal part also heats up correspondingly.
Both in the case of switches with a bimetal part through which current flows and in the case of switches with a bimetal part which is free from current, the switchover temperature is decisive for the safety function provided by the switch. The switching temperature must assume different values for different applications, but these values may only fluctuate within narrow limits in order to provide the desired safety.
Against this background, great attention is paid in the design of such temperature-dependent switches to maintaining the transition temperature.
At the same time, temperature-dependent switches with a bimetal part through which current does not flow are preferred, since they have a more constant switchover temperature. One reason for this is that the bimetal part is free from mechanical forces in the closed position, so that it is exposed to far lesser ageing processes than a bimetal part which in the closed position has to provide the contact pressure, which in the case of the other designs is undertaken by the spring snap-action part.
In particular in the case of bimetal parts through which current flows, the aforementioned creeping phase is disadvantageous, since the bimetal part stretches unpredictably in the creeping phase, causing the contact pressure to subside. This may lead to undesired contact chatter, and consequently to undesired contact erosion.
In order to overcome these problems, bimetal parts through which current flows are provided with indentations which for the most part suppress the creeping phase. These indentations ensure that the linear expansions of the two metal layers compensate for one another below the desired transition temperature. However, this leads to mechanical stresses within the bimetal parts, which in turn has adverse effects on the ageing process.
These problems do not occur in the case of the loosely inserted bimetal parts, since with them it is not necessary to suppress the creeping phase.
However, the variants of switches with a bimetal disc and a spring snap-action disc have the disadvantage that the bimetal disc and the spring snap-action disc have to be newly made to match one another with respect to their mechanical and electrical properties each time switches with different transition temperatures or different admissible operating currents are to be designed.
A further disadvantage in the case of switches with a spring snap-action disc and a bimetal disc is the large number of required structural elements, which also results in an overall height which may be problematical in certain applications.
DE 1 590 324 A discloses a bimetal part for a temperature-dependent switch that is formed as an elongated rectangle and is fixedly restrained at its one narrow end, while at its other narrow end there is a movable contact part which interacts with a fixed contact part in such a way that, when the switch is closed, the operating current of the appliance to be protected flows through the bimetal part and the two contact parts that are then in contact with one another.
The longitudinal sides of the bimetal part are folded over in such a way that the bimetal part is double-layered over about a quarter of its width on each of both longitudinal sides. Between the movable contact part and about half the length of the bimetal part, the upper layer of the double-layered longitudinal sides has been removed by punching out rectangles, which each extend over about one quarter of the width of the bimetal part. This has the effect of forming in the lower layer single-layered side webs, which between them delimit a middle web in the upper layer which takes up half the width of the bimetal part. The side webs are shortened by v-shaped stamping, so that the middle web curves convexly.
If the temperature is increased, the middle web bends counter to the bending of the rest of the bimetal part, therefore snaps through between the side webs. In this way it is intended to reduce the temperature interval within which the bimetal part snaps over between its low-temperature position and its high-temperature position.
The partly single-layered and partly double-layered structure of the known bimetal part and the shortening of the side webs have the effect that the actuating forces in the middle web and in the side webs vary greatly. Furthermore, the structure is mechanically complex and is weakened in its strength by the two punched-out rectangles.
This has the effect that the known bimetal part cannot be set exactly with respect to its transition temperature, the transition temperature not being stable in the long term because of the mechanically asymmetrical loads.
Furthermore, the known bimetal part can only be used as a bimetal spring which is restrained at one end and through which current flows, which involves the disadvantages described above.
U.S. Pat. No. 2,249,837 A describes a similar bimetal part. The known bimetal part is formed in a single-layered manner as an elongated rectangle and is fixedly restrained at its one narrow end, while at its other narrow end it bears a movable contact part, which interacts with a fixed contact part in such a way that, when the switch is closed, the operating current of the appliance to be protected flows through the bimetal part.
The bimetal part is divided by two slits running in the longitudinal direction into a middle web and two outer webs, the webs merging with one another in one piece at the narrow ends of the bimetal part. The bimetal part is deformed by bending and heat treatment in such a way that the middle web is curved down more than the two outer webs.
By adjusting the relative height of the fixed contact part in relation to the restrained narrow end of the bimetal part, the curvature of the middle web is adjusted further in comparison with the bending of the outer webs, whereby the opening temperature of the temperature-dependent switch equipped with the bimetal part is changed.
This known bimetal part can also only be used as a bimetal spring which is restrained at one end and through which current flows, which involves the disadvantages described above. Furthermore, the opening temperature must be set by subsequent adjustment work, which is likewise disadvantageous.
As a result of the different curvature of the middle web on the one hand and the side webs on the other hand, the actuating forces in the middle web and in the side webs vary greatly. This has the effect that in the case of the known part the transition temperature is not stable in the long term because of the mechanically asymmetrical loads.